Advice: you probably can prove that the other dog owner is 100% responsible (liable) for this accident.
Your dog was fenced in, and the other party’s dog trespassed. The natural tendency of any dog would be to defend its territory. Therefore you probably can prove that the other dog owner is 100% responsible (also referred to as “liable” or “at fault”) for this accident.
For the sake of thorough preparation, however, let’s see whether the other guy can raise any defenses. At the very least, you should be aware of this possibility so that you can be ready to counter it.